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Jump to navigationJump to searchCreated page with 'File:lighterstill.jpgright|frame|<center>[[Light takes about 8 minutes,19 seconds to reach Earth.</center>]] A '''light-year''' or light year (...'
[[File:lighterstill.jpg]][[File:Sun_to_Earth.jpg|right|frame|<center>[[Light]] takes about 8 minutes,19 seconds to reach [[Earth]].</center>]]
A '''light-year''' or light year (symbol: ly) is a unit of length, equal to just under 10 trillion (i.e. 1013) kilometres. As defined by the [http://en.wikipedia.org/wiki/International_Astronomical_Union International Astronomical Union] (IAU), a light-year is the distance that light travels in a [[vacuum]] in one Julian year.
The light-year is often used to [[measure]] distances to stars and other distances on a [[Galaxy|galactic]] scale, especially in non-specialist and [http://en.wikipedia.org/wiki/Popular_science popular science] publications. The preferred unit in [http://en.wikipedia.org/wiki/Astrometry astrometry] is the [http://en.wikipedia.org/wiki/Parsec parsec], because it can be more easily derived from, and compared with, [[observation]]al [[data]]. The parsec is defined as the distance at which an object will appear to move one [http://en.wikipedia.org/wiki/Arcsecond arcsecond] of [http://en.wikipedia.org/wiki/Parallax parallax] when the observer moves one [http://en.wikipedia.org/wiki/Astronomical_unit astronomical unit] perpendicular to the line of sight to the [[observer]], and is [[equal]] to approximately 3.26 light-years.
The related unit of the light-month, roughly one-twelfth of a light-year, is also used occasionally for approximate measures.
==Numerical value==
[[File:Speed_of_light_from_Earth_to_Moon.gif|center|frame|<center>Speed of light from [[Earth]] to the Moon.</center>]]
A light-year is equal to:
* exactly 9,460,730,472,580.8 km (about 10 Pm)
* about 5,878,630,000,000 miles (about 6 trillion miles)
* about 63,241.1 astronomical units
* about 0.306601 parsecs
The figures above are based on a [http://en.wikipedia.org/wiki/Julian_year_(astronomy) Julian year] (not Gregorian year) of exactly 365.25 days (each of exactly 86,400 SI seconds, totalling 31,557,600 seconds)[4] and a defined speed of light of 299,792,458 m/s, both included in the IAU (1976) [http://en.wikipedia.org/wiki/Astronomical_constant System of Astronomical Constants], used since 1984. The DE405 value of the astronomical unit, 149,597,870,691 m, is used for the light-year in astronomical units and parsecs.
[[Category: Astronomy]]
A '''light-year''' or light year (symbol: ly) is a unit of length, equal to just under 10 trillion (i.e. 1013) kilometres. As defined by the [http://en.wikipedia.org/wiki/International_Astronomical_Union International Astronomical Union] (IAU), a light-year is the distance that light travels in a [[vacuum]] in one Julian year.
The light-year is often used to [[measure]] distances to stars and other distances on a [[Galaxy|galactic]] scale, especially in non-specialist and [http://en.wikipedia.org/wiki/Popular_science popular science] publications. The preferred unit in [http://en.wikipedia.org/wiki/Astrometry astrometry] is the [http://en.wikipedia.org/wiki/Parsec parsec], because it can be more easily derived from, and compared with, [[observation]]al [[data]]. The parsec is defined as the distance at which an object will appear to move one [http://en.wikipedia.org/wiki/Arcsecond arcsecond] of [http://en.wikipedia.org/wiki/Parallax parallax] when the observer moves one [http://en.wikipedia.org/wiki/Astronomical_unit astronomical unit] perpendicular to the line of sight to the [[observer]], and is [[equal]] to approximately 3.26 light-years.
The related unit of the light-month, roughly one-twelfth of a light-year, is also used occasionally for approximate measures.
==Numerical value==
[[File:Speed_of_light_from_Earth_to_Moon.gif|center|frame|<center>Speed of light from [[Earth]] to the Moon.</center>]]
A light-year is equal to:
* exactly 9,460,730,472,580.8 km (about 10 Pm)
* about 5,878,630,000,000 miles (about 6 trillion miles)
* about 63,241.1 astronomical units
* about 0.306601 parsecs
The figures above are based on a [http://en.wikipedia.org/wiki/Julian_year_(astronomy) Julian year] (not Gregorian year) of exactly 365.25 days (each of exactly 86,400 SI seconds, totalling 31,557,600 seconds)[4] and a defined speed of light of 299,792,458 m/s, both included in the IAU (1976) [http://en.wikipedia.org/wiki/Astronomical_constant System of Astronomical Constants], used since 1984. The DE405 value of the astronomical unit, 149,597,870,691 m, is used for the light-year in astronomical units and parsecs.
[[Category: Astronomy]]
